The present invention relates generally to methods and apparatus for evaluating Stoneley waves, and more particularly relates to new methods and apparatus for evaluating earth formations through use of such waves and for determining the properties of earth formations, such as permeability, through evaluation of Stoneley wave data.
Interfacial waves are waveforms generated at fluid interfaces. The interface can be that between two fluids, as in the interface between two liquids; or it can be between a fluid and a solid, as in the case of Stoneley waves. In the context of well logging and formation analysis, Stoneley waves will traverse the interface between a generally compressible fluid column in a borehole and the slightly elastic sidewalls defining that borehole. Stoneley waves have long been recognized in well logging and seismic surveying, but typically have been regarded as undesirable manifestations which have interfered with obtaining sonic speed data associated with pressure and shear waves. Many conventional logging systems have sought to eliminate, or compensate for, the detection of Stoneley waves in evaluating formation parameters.
Some methods have, however, been proposed to establish a correlation between Stoneley waves and formation permeability. Generally, these methods have been based upon experimental observations and detailed numerical simulation to generate models. Such analyses have established the correlation of Stoneley waves to eight distinct rock, fluid and geometric parameters:
1. Well radius (R.sub.well)
2. Rock porosity (.phi.)
3. Formation permeability (k)
4. Solid shear modulus (G.sub.shear)
5. Borehole mud bulk modulus (B.sub.mud)
6. Mud density (.rho..sub.mud)
7. Pore fluid bulk modulus (B.sub.oil)
8. Pore fluid viscosity (.mu..sub.oil)
Such analyses are described, for example, in "Low-Frequency Tube Waves in Permeable Rocks," by Chang, et. al., (Geophysics, Volume 53, April, 1988); and modeling programs based upon similar analyses have been prepared by the Massachusetts Institute of Technology Full Waveform Acoustic Logging Consortium. A similar analysis is also set forth in "Permeability Estimation From Full Waveform Acoustic Logging Data," by Burns, et. al. (The Log Analyst, March-April 1988). Evaluation of formation permeability through use of Stoneley waves is also disclosed in "Predicting Relative and Absolute Variations of In-Situ Permeability From Full Waveform Acoustic Logs," by Burns (The Log Analyst, May-June 1991).
Such analyses have the drawback of being numerically oriented and extremely complex, and thus being highly disadvantageous, if not impractical, for use in evaluating formations in a commercially viable manner. For example, because of the dependence of permeability upon at least eight independent formation and fluid variables, solutions for permeability have been difficult, and have yielded less than optimally reliable results.
Accordingly, the present invention provides a new method and apparatus for evaluating formation parameters in response to Stoneley wave data. These methods are less complex than previously known analysis, and different approaches are provided which facilitate alternate methods of determining formation parameters.